1. Field of the Invention
The present invention relates to a method and an apparatus for generating a pseudo-error signal in an error rate supervisory unit which is used in a carrier wave digital modulation system, especially in a receiving unit used in an m phase PSK communication system or in a receiving unit used in a multi level QAM communication system.
2. Description of the Prior Art
The circuit quality of a PCM communication system is estimated by the error rate of a series of signal pulses. This error rate can be obtained by dividing a number of error bits generated within a predetermined time by the total number of signal pulses which are sent. If this error rate is monitored, maintenance of the circuit quality can be carried out very effectively.
In an apparatus for supervising the error rate, the following conditions are required:
(i) The error rate which is measured in the error rate supervisory apparatus must correspond precisely to a true error rate in a traffic path. PA1 (ii) The measurement of the error rate must not effect the traffic path. PA1 (iii) The measurement of the error rate must be carried out rapidly. PA1 (iv) The measurement of the error rate must be carried out without changing data which are transmitted in the system. PA1 (v) The construction of the supervisory apparatus must be simple and the cost of manufacture of the apparatus must be low. PA1 (a) A pseudo-error region is provided in a phase surface of a signal vector, and when a received signal enters into the pseudo-error region due to noise, the signal entered into the pseudo-error region is treated as a pseudo-error. PA1 (b) A phase of eye pattern discriminating clock pulses is shifted from an optimum point, and a discriminating regeneration pulse having an increased error rate is obtained. This regenerated discrimination pulse having an increased error rate is compared with the regenerated discrimination pulse obtained by using a discriminating clock pulse positioned at the optimum point. If the polarities of both regenerated discriminating pulses do not coincide, the pulses are treated as pseudo-error. PA1 (c) A level of an eye pattern discriminating threshold value is shifted from an optimum level, and a discriminating regeneration pulse having an increased error rate is obtained. This regenerated discrimination pulse, having an increased error rate, is compared with the regenerated discrimination pulse obtained by using a discriminating threshold value positioned at the optimum point. If the polarities of both regenerated discriminating pulses do not coincide, the pulses are treated as pseudo-errors. PA1 (d) Demodulated output data obtained by using a receiving filter having a very narrow bandwidth, so as to obtain the minimum error rate, is compared with other demodulator output data obtained by using a receiving filter having a wider bandwidth, for the purpose of increasing thermal noise. If the polarities of both sets of demodulated output data do not coincide, the pulses are treated as pseudo-errors.
Of the above-mentioned conditions, condition (iii) is very important. As the value of the error rate in the circuit is usually very small, a long time period would normally be required to obtain a precise error rate with few error bits. Therefore, a pseudo-error rate is actually measured by increasing the number of error bits artificially, and the true error bits are determined by measuring the pseudo-error rate, so that the time required for measuring the error rate is decreased.
Conventional methods for generating a pseudo-error signal are disclosed, for example, in the article by M. Keelty, entitled Pseudo-Error Detection Theory and Applications in QPSK 1.5 Mbps Data above FDM Voice Systems, NTC'77, pp 43:4-1.about.6, and the article by C. R. Hogge, entitled Performance Monitoring of a Digital Radio by Pseudo-Error Detection, NTC'77 pp 43:3-1.about.3. The prior art can be sumarized as follows:
In the method (a), (b) and (c), mentioned above, when the phase error of the reference carrier increases, the error rate of the system for extracting the pseudo-error increases more than the error rate (the true error rate) in the system which transmits the information. Therefore, the corresponding relation between the pseudo-error rate and the true error rate is spoiled, and the true error rate can not be estimated precisely from the pseudo-error rate. In the method (d), there is no such drawback. However, as the receiving filter has a wider bandwidth, the effect of the interference between channels is increased, so that a precise determination of the true error rate can not be obtained.